This application claims priority to French Application No. 99-14973 filed Nov. 29, 1999, which is hereby incorporated by reference.
The present invention relates to satellites and more particularly to transmitting and receiving control and telemetry signals to and from satellites via the telemetry and command link, which is also used for measuring the distance between satellites and ground stations. The link is usually referred to as the telemetry, tracking, and command (TTC) link or the telemetry, command, and ranging (TCR) link. It must be established very reliably during all stages in the life of a satellite, of which there are four:
a station injection stage, which corresponds to the period between the satellite being injected into orbit by the launch vehicle and reaching its final position;
a station-keeping stage, which corresponds to the stage in which the satellite is in normal operation;
a standby stage, which corresponds to an equipment failure and therefore does not always occur, and during which the attitude of the satellite can change; and
a deactivation stage, also referred to as orbital ejection, during which the satellite is moved to a cemetery orbit or re-enters the atmosphere.
It is important for the TTC link to be maintained throughout all the above stages, for which purpose satellites are provided with antennas. A first set of omnidirectional antennas is used during the station injection and standby stages, during both of which the payload of the satellite is not active and the attitude of the satellite can vary.
The payload of the satellite is active and the satellite retains a nominal position during its station-keeping stage. A directional antenna can therefore be used.
Both antennas are generally connected to a redundant command receiver which demodulates the signals received and usually processes low bit rates, of the order of 1 kbit/s. Subcarrier modulation is used by prior art satellites, for example NRZ/BPSK/FM modulation in the case of the ARABSAT, ASTRA, STENTOR, NILESAT, etc. satellites. This entails modulating a subcarrier using a NRZ (no return to zero) waveform and BPSK (binary phase shift keying) modulation. The subcarrier is at a frequency of the order of 8 kHz, for example, and is used to frequency modulate the carrier, which is at frequencies of around 5800 MHz-6700 MHz in the case of systems operating in the C band or at frequencies of around 12700 MHz-14800 MHz in the case of systems operating in the Ku band, for example, with a frequency excursion of the order of 400 kHz.
U.S. Pat. No. 4,691,236 describes a receiver for use in a satellite telecommunications system. To alleviate the effects of inclement weather, it proposes transmitting signals to the satellite in two modes. In a first, clear sky transmission mode, frequency modulated picture, sound and data signals are transmitted to the satellite; the picture signals are modulated at a frequency of up to 4.5 MHz and the sound and data signals are quaternary PSK modulated onto a 5.727 MHz subcarrier. In a second, standby transmission mode, only sound and data signals are transmitted, using direct frequency modulation of the carrier. It is thus possible to use all of the available band when the sky is clear and to transmit sound and data signals in the bandwidth available in standby mode. The above document suggests determining which transmission mode to use by detecting the presence of baseband synchronization signals after the carrier has been demodulated in the receiver. Video signals are present only in the clear-sky transmission mode. The above document is silent on the problem of satellite control. Also, the above document teaches modulating the subcarrier only to shift the frequency of sound and data signals in order to enable them to be frequency division multiplexed with picture signals.
WO-A-99 21287 describes a method of compensating variations in the quality of an RF link, such as a satellite link. It proposes changing from QPSK modulation to BPSK modulation if the link quality is degraded, which produces a 3 dB improvement. The bit rate is halved for the same symbol frequency.
A problem that is sometimes encountered nowadays is that of increasing satellite command bit rates, caused in particular by the increasing complexity of payloads and their increasing control demands.
An object of the invention is to solve this problem. Through minor modifications of existing receivers, it enables different bit rates to be used on the command link of a satellite in different stages of the life of the satellite. The invention avoids any need for a multiplicity of receivers and retains a low bit rate for the satellite station injection and standby stages. The link budget is maintained during these stages, despite the use of large-coverage, low-gain antenna(s). The invention also enables automatic changeover from one bit rate to the other.
To be more precise, the invention provides a method of transmitting on the command link of a satellite, the method including:
at least one stage of direct modulation of a carrier for transmission on the link at high bit rates; and
at least one stage of subcarrier modulation of said carrier for transmission on the link at low bit rates.
In one implementation, direct modulation is used during a stage in which the satellite is pointed at the Earth, such as the station-keeping stage.
In another embodiment, subcarrier modulation is used during a stage in which the satellite is not pointed at the Earth, for example during the station injection stage or the standby stage.
The invention also provides a receiver for the command link of a satellite, the receiver including:
a carrier demodulator;
a subcarrier demodulator receiving the signals supplied by the carrier demodulator; and
bit synchronization and detection means receiving the signals supplied by the carrier demodulator.
The receiver preferably also includes a switch receiving as input the signals supplied by the subcarrier demodulator and by the bit synchronization and detection means. The switch can be controlled as a function of the presence of a subcarrier in the signals supplied by the carrier demodulator.
The invention further provides a satellite including a receiver of the above kind.
The invention further provides a transmitter for the command link of a satellite, the transmitter including:
a signal generator;
a switch receiving the signals supplied by the signal generator;
a subcarrier modulator receiving the signals from one output of the switch; and
a carrier modulator receiving the signals from the other output of the switch or the subcarrier modulator.
The switch is advantageously controlled as a function of the bit rate of the signals supplied by the signal generator.